What happened
Between June and July 2021, several aircraft operating at London Heathrow experienced abnormal pitot/static system indications. The sequence of events began on 9 June 2021, when an Airbus A320-232, G-EUUO, experienced a rejected takeoff after the commander noted a significant airspeed discrepancy on the primary flight display. Shortly after, on 10 June, an Air and Airbus A320-251N, G-TTNH, experienced multiple system alerts and flight control law degradation during pushback, leading to a return to the stand. On 11 June, a Boeing 777-236, G-YMMR, also required a rejected takeoff due to non-functioning airspeed indicators during the takeoff roll.
Subsequent inspections of other aircraft revealed similar issues. A Boeing 787-9, G-ZBKJ, was found with insect eggs inside a probe, while a Boeing 777-336ER, G-STBJ, had an insect visible at the tip of its pitot probe. An Airbus A330-343, G-VKSS, also showed evidence of debris and contamination in its probes during maintenance. In one instance, a live insect was even discovered during a pre-departure inspection of an Airbus A319-131, G-DBCG.
The investigation
The AAIB investigation focused on the cause of these concentrated blockage events. Physical examinations of the debris and probes were conducted. In the case of G-YMMR, the blockage was identified as a hard cap characteristic of nests built by solitary bees or wasps. DNA analysis performed by the Natural History Museum identified two specific species involved: the Wall Mason wasp (*Ancistrocerus parietum*) and the Hairy-toothed small leafcutter bee (*Megachile pilidens*).
The investigation examined environmental factors at Heathrow, noting that during the pandemic, reduced aircraft movements, lower noise levels, and decreased human activity created a more attractive environment for pollinators. Furthermore, a decrease in nitrogen dioxide levels led to a rise in ground-level ozone, which can influence insect foraging and nesting behavior.
Findings
- The primary cause of the pitot probe blockages was nesting activity by certain species of wasps and bees within the probes.
- The unusual concentration of events was likely due to the reduced operational tempo at Heathrow during the COVID-19 pandemic, which allowed insects to utilize uncovered pitot probes as nesting sites.
- The presence of the Wall Mason wasp was particularly notable as its nests involve sealing tubes with clay, which can create a pressure chamber that causes anomalous airspeed readings when the pitot heaters are activated.
- Reduced levels of pollutants and noise at the airport may have influenced the success of these pollinators during this period.